This invention relates to switched capacitor circuits or networks and more particularly to a switched capacitor circuit replacement for a source resistor.
There is currently much interest in the simulation of resistors with switched capacitor circuits so that filters and networks can be implemented in fully integrated circuit form. Switched capacitor source resistors are particularly useful in LC ladder filters. Switched capacitor source resistors are generally described in the references "Switched-Capacitor Filters Using Floating-Inductance Simulation Circuits" by M. S. Lee, Electronics Letters, Sept. 27, 1979, Vol. 15, No. 20, pages 644-645; "Switched-Capacitor Filter Design Using the Bilinear z-Transform" by G. C. Temes, et al., IEEE Transactions on Circuits and Systems, Vol. 25, No. 12, December, 1978, Pages 1039-1044; and "Basic Principles of Switched-Capacitor Filters Using Voltage Inverter Switches", by A. Fettweis, AEU, 1979, Vol. 33, Pages 13-19.
A number of previously existing switched-capacitor resistors that are useful as source resistors have a direct connection of a capacitor between input and output terminals thereof at some time during a cycle of operation. This provides feedthrough of an analog input signal directly to the output of the simulated resistor which degrades the operation of a discrete-time filter connected to it. This condition has been overcome in the past by inserting a separate sample-hold circuit before the circuit that simulates the source resistor. In the past, this has necessitated multiple circuits with the requisite active and passive elements thereof.
An object of this invention is the provision of an improved switched-capacitor source resistor. Another object is the provision of a switched-capacitor circuit that simultaneously simulates a source resistor and performs a sample-hold function on analog input signals.